Women born between 1981 and 2000 have salaries that are 5% lower on average, while women born between 1966 and 1980 were paid 9% less than men when they were in their 20s.

Amanda Goodall, a senior lecturer at Cass Business School in London, said progress has been made because today's young women are better equipped to successfully negotiate salaries than previous generations.

The starter salary gap is also somewhat smaller than many women expect. A recent study found that female U.K. students expect to make 14% less than their male peers in their first jobs.

The bad news? Today's young women can still expect to be underpaid throughout their careers.

"Small hourly pay gaps quickly grow into large lifetime pay penalties that can leave women ... worse off over the course of their careers," said Laura Gardiner of the Resolution Foundation.

Having children still carries a sizable pay penalty.

"Training, progression and promotion are much harder to come by when working part time, which many women with children either choose to do or feel they have to because of high childcare costs," Gardiner added.